The present invention relates to granulated particles suitable as a raw material for magnetic particles for magnetic recording and a process for producing the granulated particles.
Miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording have recently shown a remarkable progress. Especially, video tape recorders (VTR) have conspicuously spread and the development of miniaturized and lightweight VTR's for longer-time recording have been conducted.
With this development, magnetic recording media such as a magnetic tape and magnetic disk have been strongly required to have a higher performance such as a higher recording density, a higher sensitivity and higher output characteristics. In order to satisfy the above-described demands on magnetic recording media, the magnetic particles are required to have a high coercive force and the distribution of the coercive force among particles must be as narrow as possible. Namely, in order to achieve a higher sensitivity and higher output characteristics, it is required that the magnetic particles have a higher coercive force. This fact is described in, for example, Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Kabushiki Kaisha Sogo Gijutsu Center (1982) on page 310, "Since the development of the performance of a magnetic tape is aimed at a higher sensitivity, a higher output . . . , it is an important point to enhance the coercive force . . . of acicular .gamma.-Fe.sub.2 O.sub.3 "
In order to enhance the output of a magnetic recording medium, it is necessary that S.F.D.(Switching Field Distribution) value is small, as seen from the description in Japanese Patent Application Laid-Open (KOKAI) No. 63-26821 (1988): "FIG. 1 shows the relationship between the S.F.D value and the recording and reproducing output of the magnetic disc . . . The relationship between the S.F.D value and the recording and reproducing output is linear, as is obvious from FIG. 1, which proves that the use of ferromagnetic particles having a small S.F.D. value enhances the recording and reproducing output. That is, in order to increase the recording and reproducing output, the S.F.D. value is preferably as small as possible. In order to obtain a higher output more than the ordinary one, it is necessary that the S.F.D. value is not more than 0.6." In order to have a small S.F.D. value the distribution thereof among particles is required to be narrow.
As well known, the coercive force of magnetic particles largely depends upon the shape anisotropy, crystalline anisotropy, strain anisotropy or exchange thereof.
Magnetic iron oxide particles such as acicular magnetite (FeOx.multidot.Fe.sub.2 O.sub.3, 0.ltoreq.x&lt;1) particles and acicular maghemite particles, and acicular magnetic iron based alloy particles which are used as magnetic particles for magnetic recording have a comparatively high coercive force by utilizing the anisotropy derived from the shape of each particle.
These known magnetic iron oxide particles are produced by reducing as a raw materials acicular goethite particles or acicular hematite particles obtained by heating and dehydrating acicular goethite particles, in a reducing gas such as hydrogen so as to produce acicular magnetite particles, or further oxidizing the acicular magnetite particles in air so as to produce acicular maghemite particles.
The known Co-modified, or Co and Fe.sub.2+ -modified acicular magnetic iron oxide particles are obtained by dispersing acicular magnetite particles or acicular maghemite particles as the precursor particles in an alkali suspension containing cobalt hydroxide or an alkali suspension containing cobalt hydroxide and ferrous hydroxide, and heat-treating the dispersion.
The most important process for influencing various properties of the magnetic iron oxide particles obtained is a heat-treatment process. As the heat-treating apparatus used in the heat-treatment process, a rotary furnace for heat-treating the raw material while rotating it and a fluidized-bed furnace for heat-treating the raw material while fluidizing it, are known.
Also, magnetic iron based alloy particles are obtained by using, as the raw material, acicular goethite particles, acicular hematite particles obtained by dehydrating acicular goethite particles at a temperature of 150.degree. to 280.degree. C. or acicular hematite particles having a high density obtained by heating acicular goethite particles to 300.degree. to 850.degree. C. in a non-reducing atmosphere, heat-treating these particles in a stream of a reducing gas such as hydrogen gas and then subjecting the obtained particles to surface-oxidizing treatment. The most important process for influencing various properties of the magnetic iron based alloy particles obtained is a process of heat-treating a raw material particles. As the heat-treating apparatus, a rotary reducing apparatus for the raw material particles while rotating, a fluidized-bed reducing apparatus for heat-treating the raw material particles while fluidizing, and a fixed-bed reducing apparatus for heat-treating the raw material particles in a fixed state, etc. are known.
When magnetic particles are produced by heat-treating acicular goethite particles or acicular hematite particles as the raw material in a rotary furnace or fluidized furnace, or a rotary reducing apparatus or fluidized-bed reducing apparatus, respectively, the reaction thereof proceeds uniformly, because the rotation or fluidization of the raw material particles makes the heat-treating or reducing atmosphere uniform. However, since collision or friction is caused between the raw material particles or between the raw material particles and the wall of the reactor due to the rotation or flow of the raw material particles, sintering between the particles and the lost of shape of a particle are apt to be caused in the heat-treating process or heat-reducing process, so that the magnetic characteristics are deteriorated. On the other hand, in the case of using a fixed-bed reducing apparatus, since the raw material particles is reduced in an approximately static state, sintering between the particles and the lost of shape of particles which are caused by the collision or friction of the raw material particles due to rotation or fluidization are unlikely to be produced. In the case of heat-reducing the particles in a fixed-bed reducing apparatus, in this way, particles granulated in various means are used as the raw material in order to prevent the raw material particles from being fluidized against the flow of a gas stream.
Methods of producing magnetic iron based alloy particles by using a fixed-bed reducing apparatus are disclosed in Japanese Patent Publication Nos. 61-36048 (1986), 1-52441 (1989), 1-52442 (1989), 1-52443 (1989) and 1-52444 (1989), Japanese Patent Application Laid-Open (KOKAI) No. 54-62915 (1979), etc.
In case of heat-treating or heat-reducing acicular goethite particles or acicular hematite particles, it is necessary to use granulated particles which produce little fine powder due to the collision or friction between the raw material particles or between the raw material particles and the wall of the reactor.
As examples of a method of granulating the raw material in advance, a method of supplying finely pulverized .alpha.-Fe.sub.2 O.sub.3 particles and water sprinkled through a spray to a granulation a predetermined ratio to obtain granulated particles (Japanese Patent Application Laid-Open (KOKAI) No. 63-88807 (1988)), and a method of dispersing fine iron oxide particles in water, compressing and dehydrating the dispersion by a filter-press to a water content of 60 to 80 wt % so as to obtain pellets (Japanese Patent Application Laid-Open (KOKAI) Nos. 57-54205 (1982) and 57-116706 (1982)) are known.
To state this more concretely, Japanese Patent Application Laid-Open (KOKAI) No. 57-54205 (1982) discloses a method of producing a magnetic metal fine powder containing iron as the main ingredient comprising the steps of: dispersing in water a metal compound powder containing iron as the main ingredient; compressing and dehydrating the resultant dispersion to a water content of 60 to 80 wt % so as to obtain pellets; and reducing the pellets at a temperature of 300.degree. to 600.degree. C. by a reducing gas such as hydrogen.
Japanese Patent Application Laid-Open (KOKAI) No. 57-116706 (1982) discloses a method of producing a magnetic metal powder comprising the steps of: coating the surfaces of metal compound particles containing iron oxyhydroxide or iron oxide as the main ingredient with a silicon compound; granulating the coated metal compound particles into pellets; and heat-treating the pellets in a reducing atmosphere.
As the means for granulating the metal compound particles coated with the silicon compound, a method of dispersing the particles in water, and compressing and dehydrating the dispersion by a filter-press to a water content of 60 to 80 wt % so as to obtain pellets; a method of adding water to the above-mentioned particles to a water content of 35 to 45 wt % kneading the mixture, and extruding the resultant mixture into masses by an extruder; a method of compacting the particles in a dried state by a compression of 200 to 1000 kg/cm.sup.2 by a tablet machine into pellets, etc. are proposed in Japanese Patent Application Laid-Open (KOKAI) No. 57-116706(1982).
Japanese Patent Application Laid-Open (KOKAI) No. 63-88807(1988) discloses a method of producing a magnetic iron powder comprising the steps of: coating the surfaces of iron oxyhydroxide particles with a magnetism-adjusting component and/or shape-retaining component; heat-treating the coated particles in the reducing atmosphere; pulverizing the heat-treated particles; granulating the pulverized particles by using water as a binder; drying and reducing the granulated particles.
In any of the above-described are granulated by using water as a binder.
Industrially and economically advantageous production of magnetic particles having a high coercive force, a large saturation magnetization and narrow distribution of the coercive force and the saturation magnetization among particles is now in the strongest demand. However, if magnetic particles are produced by any of the above-described related art using the granulated particles produced by a rotary furnace, rotary reducing apparatus or fixed-bed reducing apparatus, the distribution of the coercive force and the saturation magnetization among the obtained particles become so wide that it is impossible to obtain magnetic particles which satisfy the above-described demand for various properties.
It is because the granulated particles which are likely to produce a fine powder are used as the raw material that the distribution of the coercive force and the saturation magnetization among the magnetic particles obtained become wide when the granulated particles are heat-treated in a rotary furnace or heat-reduced in a fixed-bed reducing apparatus.
Since water is used as a binder for producing the granulated particles in the art, the granulated particles have a low strength. Under the condition in which the linear speed of blowing gas such as oxidizing gas, reducing gas and inert gas is comparatively large at the time of heat-treating or heat-reducing so as to enhance the reducing and oxidizing efficiency or the reducing efficiency, it is often the case that fine powder is produced by friction between granulated particles having a low strength when weak vibration is applied thereto, and that fine powder is produced when the raw material is charged (transferred) into a heat-treating furnace or a fixed-bed reducing apparatus, or when the raw material is heated and expanded. In this way, when granulated particles having a low strength are used as the raw material and heat-treated in a rotary furnace or heat-reduced in a fixed-bed reducing apparatus, the fine powder produced is existent locally in the bed and the pressure loss at that portion becomes large, thereby producing a channel of the blowing gas, which makes the progress of reduction and oxidization or reduction of the particles non-uniform, thereby deteriorating the reducing and oxidizing efficiency or the reducing efficiency. As a result, the magnetic iron oxide particles obtained disadvantageously have a wide distribution of the magnetic properties (coercive force and saturation magnetization) among the particles.
On the other hand, a method of using an organic material as a reducing agent in the production of magnetic iron based alloy particles is proposed in Japanese Patent Application Laid-Open (KOKAI) No. 55-82408 (1980).
More specifically, Japanese Patent Application Laid-Open (KOKAI) No. 55-82408 (1980) discloses a method of producing an iron powder and an iron-cobalt alloy powder for a magnetic tape, the method comprising the steps of: preparing acicular iron oxide particles or cobalt-containing iron oxide particles as the raw material; adhering an organic material having adhesiveness to the surfaces of the particles of the raw material; and heating the resultant particles in a reducing gas, in a inert gas or in a vacuum so as to reduce the raw material with the organic material, thereby obtaining an iron powder or an iron-cobalt alloy powder retaining the acicular shape of the raw material.
As the preferred organic material is described therein an inexpensive organic material which decomposes at a low temperature and produces no corrosive gas such as cellulose acetate, ethyl cellulose, methyl cellulose, styrene, vinyl alcohol, vinyl acetate, starch and dextrin. In the above-described method, an organic material adheres to the surfaces of the particles of the raw material, and the amount thereof is 5 to 50 wt % based on the total amount of particles as the raw material.
As is obvious from the description in the Japanese Patent Application Laid-Open (KOKAI) No. 55-82408 (1980), of "In the method of the present invention, iron oxide powder is reduced into an iron powder by producing CO, CO.sub.2 or H.sub.2 O combining a carbon atom and a hydrogen atom, which are components of the organic material adhered to the surfaces of the iron oxide particles, with an oxygen atom of the iron oxide particles when heated . . . ", an organic material is used as a reducing agent.
Although Japanese Patent Application Laid-Open (KOKAI) No. 55-82408 (1980) discloses iron oxide particles coated with an organic material, it is silent on granulated iron oxide particles and a granulating method. In addition, since the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 55-82408 (1980) is a method of using an organic material as a reducing agent, the object of the use of the organic material is completely different from that of the present invention, and there is no description which motivates the use of the compounds according to the present invention.
That is, it is required to provide magnetic particles for magnetic recording which have a high coercive force and as narrow distribution of the coercive force and the saturation magnetization among particles as possible.
As a result of various studies undertaken so as to achieve the above-described requirements, it has been found that by adding to a suspension composed of acicular ferric hydroxide oxide particles or acicular iron oxide particles and water, not less than 0.1 part by weight and less than 5.0 parts by weight of water- or hot water-soluble semisynthetic starch or semisynthetic cellulose based on 100 parts by weight of the particles in the suspension; stirring the resultant suspension; compressing and dehydrating the suspension to obtain a cake; and granulating the cake, the obtained granulated particles produces little fine powder even in the case of using a rotary furnace or a fixed-bed reducing apparatus and, hence, produces no channel of the blowing gas in the bed at the time of heat-treating or heat-reducing, thereby making the progress of reduction and oxidization or reduction of the granulated particles uniform, and as a result, such granulated particles are suitable as the raw material for magnetic particles for magnetic recording which have a high coercive force, a large saturation magnetization, and narrow distribution of the coercive force and the saturation magnetization among particles. The present invention has been achieved on the basis of this finding.